The 1999 Jeep Cherokee employs an interesting idler pulley assembly. What you need to do is:
1. Remember the belt routing, it's not printed under the hood.
2. Loosen the 15mm hex nut on the idler pulley
3. There's a bracket that the idler pulley is connected to and a 15mm tensioning bolt to turn to release the tension off of the belt.
Jeep Cherokees seem to like Premium belts best.